Implement universal helper function for pagination based on cursors

[MarshalX]

my thoughts about impl: #93 (comment)

[steve-mavens]

Since there might be many calls, and in general the user might not be able to tell in advance how many: if/when you go ahead with this I'd suggest either a parameter to limit how far it goes, or else that the helper function returns an iterator. That can make more calls on demand as the caller consumes the results.

There's different ways the iterator could work, though. One is that it could be an iterator of Response objects (so the caller can/must check for errors at each page). Or, if the helper handles "enough" error-checking, then when calling search_posts with paging perhaps it could return an iterable of PostView and so on. I don't know how difficult it would be to get the type annotations right for that, though.